Recent information has established that salt, water, and small organic molecules can cross epithelia without traversing cell membranes, that this passage is rate-limited by membrane contacts or "tight" junctions between adjacent cells, and that this passive permeability is adjusted by the magnitude, direction and the solute employed in establishing a transepithelial osmotic gradient. Much of this information has been derived from studies of the toad urinary bladder, a functional analog of the distal mammalian nephron. It is proposed that, with an extension of electrophysiologic and electron microscopic techniques that are available for examination of epithelia, a quantitative analysis of the functional significance of extracellular transport will be carried out. Specifically, attention will be directed to the means by which the electrical conductance of this permeation pathway is adjusted. The principal methods to be employed are those of membrane electrophysiology and transmission electron microscopy with serial analysis of individual preparations during and after exposure to variously created osmotic gradients. Correlation of measurements of short-circuit current and transmural conductance with a quantitative description of the osmotically-induced conformational changes in the structure of cell junctions will be used to evaluate the characteristics of the extracellular pathway in terms of its solute specificity, its importance in passive ion transfer, and its influence on the active transport of sodium by this epithelium.